In the rolling of a pipe in a pipe-rolling mill, the measurement of wall thickness is important to determine the uniformity of the product and the achievement of the particular rolling requirements.
For this reason a laser ultrasonic method has been developed for measuring the wall thickness, both for the purposes of controlling the rolling mill and for the purpose of determining the quality of the rolled product.
In this laser ultrasonic process, an excitation laser trains a beam of laser radiation against the surface of the pipe to launch an ultrasonic pulse through the thickness of the pipe. The ultrasonic signal is reflected on the opposite wall of the pipe and the reflected ultrasonic signal returns to the surface at which the exciting laser was effected. Using an illumination laser to detect the ultrasonic signal and an interferometer for analyzing the ultrasonic-modulated laser carrier beam, an output can be provided to an evaluating unit or computer which can determine the transit time for the ultrasonic wave to cross the thickness of the pipe and calculate from that transit time and the speed of sound in the material of the pipe, the wall thickness.
In earlier systems the interferometer consisted of two mirrors which could be positioned by means of a linear actuator or effector relative to one anther at a predetermined distance. For control of this distance and the spacing of the mirrors, a photodiode measured the light transmitted by the mirror and from the measured signal a signal was applied directly or indirectly to a controller which, in turn, operated the linear actuator.
In many fields, seamless pipe of steel is required and to manufacture such pipe a cylindrical billet is pierced in an inclined roll mill by an axial piercing mandrel. The pierced billet is then rolled in pipe-rolling mill stands in a rolling line to a seamless pipe in part over the piercing mandrel at least in an early stage and subsequently without the piercing mandrel in later stages. Such a method is described, for example, in EP 0 940 193 A2.
Using stretch-reduction rolling and reducing rolling and rolling to size, seamless steel pipe can be produced with desired wall thicknesses in a plurality of such mill stands. In each of the mill stands, rolls are journaled so that each engages the periphery of the pipe over a certain limited peripheral segment and thus, for example, a roll stand can include a multiplicity of rolls, for example, three, positioned so that the pipe is contacted over the entire periphery by the rolls. The pipe is then rolled to a reduced diameter in each mill and given a true pipe shape.
The pipe following the rolling operation should have an ideal shape, that is the cylindrical outer contour and the cylindrical inner contour should form two concentric circles in each cross section through the pipe taken axially along the length of the pipe. In practice however, there are tolerances in the finished pipe so that a certain eccentricity of the circular contour of the inner periphery relative to the circular contour of the outer periphery always occurs.
A defining quality parameter for the production of the pipe by rolling is the pipe wall thickness which is measured in the course of the production process as has been described, for example, and is monitored. Around the periphery the variation in such thickness should not exceed a certain limit and along the length of the pipe, the same limit in the deviation of the wall thickness should apply.
The laser ultrasonic thickness measurement system mentioned previously is described for example in U.S. Pat. No. 5,137,361 and WO 00/63,641. The interferometer there described is a confocal Fabry-Pérot interferometer and the evaluation unit, e.g. the computer, calculates the transit time through the thickness of the wall of the pipe from the time spacing between the applied and reflected ultrasonic signal or between two echoes of the echo sequence which is produced, using the known speed of sound in the material from which the pipe is composed.
The Fabry-Pérot interferometer has two mirrors which are positioned relative to one another at a predetermined spacing and can be adjusted in this spacing by a piezo linear actuator, i.e. a crystal which displaces one of the mirrors linearly in response to an electrical signal applied to that crystal. Since it is important that the spacing of the two mirrors be exact (in the nm range), so that an effective measurement can be made of the wall thickness, the control of the spacing must use the steep flank of the intensity curve in the intensity versus frequency plot of the light signal. For this purpose, in the system of U.S. Pat. No. 5,137,361, a control circuit is provided for applying a control signal to the piezo actuator, using two photo diodes, one of which measures the light supplied to the interferometer and the other of which measures the light transmitted thereby.
It has been found in practice that this approach does not always provide a satisfactory wall thickness measurement of a pipe under the rough conditions which prevail in a pipe-rolling mill. Generally the earlier systems have not only failed to stabilize the Fabry-Pérot interferometer sufficiently, but they have also not been able to satisfactorily guarantee a reproducible result in rolling mill applications.